The present invention relates to steerable bottom hole assemblies when used for underreaming or enlarging a section of a wellbore.
Steerable bottom hole assemblies include a positive displacement motor (PDM) or xe2x80x9cmud motorxe2x80x9d which has a bend angle that allows a drilling operator to build at a desired rate when xe2x80x9cslidingxe2x80x9d the non-rotating motor housing. The bend angle of the bent sub or bent motor housing, in combination with the contact points of the bottom hole assembly (BHA) with the well bore wall, thus largely determines the build rate for drilling the curved borehole section. When the drill string and the motor housing are rotated, the drilling operator drills a straight or tangent section of the borehole. The rate of penetration (ROP) of the bit drilling through the formation is frequently significantly less when xe2x80x9cslidingxe2x80x9d to build rather than rotating the bottom hole assembly from the surface to drill the straight borehole sections. Since ROP translates to time and money, drilling operators prefer to maximize the time when the BHA is rotated for drilling straight, and minimize the time of building. For various reasons, high build rates are generally preferred, but the bend angle is preferably low to minimize problems when the BHA is rotated. A steerable bottom hole assembly is disclosed in U.S. Pat. No. 6,269,892. This patent discloses the benefits of a steerable bottom hole assembly with a long gauge bit, i.e., a bit having a gauge section with an axial length of at least 75% of the bit diameter.
Underreaming tools within the bottom hole assembly are used to enlarge a section of the well bore below a restriction. Advantages of underreaming are disclosed in an article xe2x80x9cSimultaneous Drilling and Reaming Saves Rig Timexe2x80x9d, Oil and Gas Journal, Nov. 27, 1989. Conventional underreaming tools use three or more cutting arms that are moved outward in response to fluid pressure within the tool. A reamer designed for hole opening while drilling is the NBR reamer offered by Security DBS, a Halliburton Company.
When reamers are used in a conventional steerable bottom hole assembly immediately above the bit and below the motor housing, the bottom hole assembly has very poor steerability. More particularly, it is difficult to accurately predict the build rate when sliding, regardless of the positioning of stabilizers above the reamer. Because of high vibration, the azimuth of the curved borehole formed while sliding may be inaccurate. Moreover, borehole quality is generally poor due to high vibration, which prohibits the motor housing from xe2x80x9cholding steadyxe2x80x9d in the well.
Because of the disadvantages of a steerable bottom hole assembly and reamer combination discussed above, operators have used bi-center bits rather than reamers for performing the underreaming operation. Bi-center bits are discussed in xe2x80x9cApplication of Bi-Center Bits in Well-Deepening Operations, IADC/SPE 19921, page 131. While bi-center bits may be a better solution than a reamer in some applications, significant problems remain when using a bi-center bit. Vibration due to the bi-center bit is significant, and the bottom hole assembly is also difficult to steer. Vibrations are particularly high when drilling relatively hard formations. High vibration not only increases the likelihood of failure in the bottom hole assembly, but is generally indicative of poor borehole quality. High vibration typically results in excessive bit wear. Drilling operators are constantly seeking techniques which will result in better hole quality when drilling with a steerable downhole motor. The bottom hole assembly also should have good steerability and preferably be able to drill at a relatively high ROP.
The disadvantages of the prior art are overcome by the present invention, and an improved steerable bottom hole assembly and method are hereafter disclosed for underreaming a section of borehole with relatively low vibration and high borehole quality.
A steerable bottom hole assembly according to the present invention is used for drilling both a curved section and a straight section of the borehole. The bottom hole assembly includes a downhole positive displacement motor having a motor housing and a bend angle. The motor housing encloses a shaft or rotor offset at a selected bend angle from a central axis of an upper housing, which encloses the power section of the motor. The motor is powered by fluid to rotate the bit when sliding to drill the curved section of the borehole, and the bottom hole assembly including the motor is rotated from the surface to drill the straight section of the borehole.
A gauge section is provided directly above the bit and has a substantially uniform diameter cylindrical surface with an axial length of at least 75% of the bit diameter. A reamer is positioned directly above the bit and has a reamer diameter significantly greater than the bit diameter for drilling an enlarged section of the borehole compared to the restricted diameter through which the bottom hole assembly has passed.
It is an object of the present invention to provide a steerable bottom hole assembly which is useful for underreaming operations and results in relatively high borehole quality compared to prior art techniques.
It is a feature of the present invention that the bend-to-bit distance is less than 15 times the bit diameter, preferably less than the 12 times the bit diameter, thereby obtaining a relatively high build rate for a low angle bend in the motor. To provide this low bend-to-bit distance, the lower end of the motor includes a pin connection at the end of the shaft or motor for mating engagement with a box connection at the upper end of the reamer.
It is a feature of the present invention that the positive displacement motor is preferably run slick, i.e., with no stabilizers for engaging the wall of the well bore.
It is another feature of the present invention to provide a bottom hole assembly with a second reamer repositioned above the bend in the downhole motor.
In a preferred embodiment, the bend in the bottom hole assembly has a bend angle of less than 3xc2x0, and the gauge section has an axial length of at least 90% of the bit diameter.
A related feature of the present invention is to provide a method of forming a subterranean borehole utilizing a steerable bottom hole assembly as discussed above, including a reamer below the downhole motor for enlarging the borehole diameter substantially beyond the bit diameter. A gauge section is provided between the bit and the reamer to add stability to the BHA. The motor is powered with fluid to rotate the bit while the motor housing is slid to drill a curved section of the borehole, and the bottom hole assembly is rotated from the surface to build a straight section of the borehole.
A significant advantage of the present invention is that the bottom hole assembly does not require specially made components. Instead, each of the components of the bottom hole assembly may be selected by the operator as desired to achieve the objectives of the invention.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.